Sulfur hexafluoride is pretty awesome. It is a gas six times denser than regular air, and thus you can actually make tinfoil "boats" float on it. It also makes your voice deep when you breathe it in.
Let's have a sea of it. To prevent the entire planet from becoming a barren wasteland, make it a single sea about the size of the Black Sea, with an island the size of Cyprus in the middle. It's surrounded by tall mountains so there's little wind that would blow it empty. When, and with what technology, could the first people reach the island?
Sulfur hexafluoride is dense, but not dense enough for a regular boat to float on. You could not walk to the island either, because the gas will displace oxygen in your lungs and you will asphyxiate.
Because it is so dense, I wonder if you could navigate this sea with some kind of boat/plane hybrid. Or something similar to Da Vinci's flying machine. Could people reach this island at any time before the invention of the hot air balloon?